Southern Testing HoleBASE SI

Site investigation consultant and contractor Southern Testing was appointed to carry out investigations for a proposed scheme of more than 400 homes on the edge of Maidstone in Kent.

Maidstone sits in the Medway Valley, which is well known for the cambering of the Hythe Formation: the movement and rotation of interbedded limestone and sandstone towards the river valley has caused fissures (cambers) to open, creating blocks of rock that rotate downslope. These cambers can be up to 5m wide, loosely infilled with clays and sands and in some instances have areas of voids.

Identifying the nature and extent of these loosely infilled and sometimes voided features was critical for the developer as they can lead to differential settlement and subsidence due to inundation. The investigation therefore aimed to identify these areas, to enable the client to design special foundations on a plot-by-plot basis and identify areas where ground improvement works, such as compaction grouting, may have been required.

“The client and the NHBC had agreed dynamic probing was the best approach,” explains Southern Testing Senior Engineer Darryl Kelly. “Probing was carried out at the corner of each plot; deeper probes were carried out at the proposed location of the borehole soakaways.”

Dynamic probing produces a lot of raw data which can be time consuming to enter, validate and assess. With thousands of probes being carried out, this represented a significant challenge to Southern Testing, given the tight programme.

“We had to identify the presence of any loosely infilled and voided features quickly and then carry out more targeted probing to determine their lateral extent,” Kelly says. “Time was of the essence: the client’s engineer needed to decide quickly if foundation designs had to be adapted or if ground improvement was needed.”

Results were provided daily using specially-designed KeyAGS data entry sheets, allowing AGS v4 data to be imported the next day into HoleBASE SI.

“Initially, HoleBASE SI was used to plot the dynamic probe results, to allow us to visually assess the blow counts and to identify any features requiring further investigation. Data was then transferred onto a large hardcopy plan,” Kelly explains.

“The results from the additional targeted probing carried out on these features was then added to the plans but with so many probes, processing the data soon became very time-consuming and wasteful – with new plans being printed out each time – so a new approach was needed.

“We first tried looking at using trigger sets on the incremental blows to highlight potential loose areas and voids,” Kelly says. “However, this proved inconclusive because ground conditions were so variable and because of the large number of results from each probe.”

Instead, the team produced a template using the HoleBASE SI Excel Extension that could calculate the cumulative blows per metre, highlighting potential loose zones and voids at each location and therefore enabling rapid assessment of each probe result.

“We used the Data Entry Feedback Tool to create .csv files which could be imported back into HoleBASE SI,” Kelly explains. “The newly-created ‘blows per metre’ column in HoleBASE SI was then set up with a trigger set, which provided a more accurate assessment of the potential ground conditions at each location. HoleBASE SI also enabled us to make a visual assessment of the extent and zoning of the features.”

The approach reduced the number of man hours spent analysing raw data significantly, he adds. “Assuming a two-and-a-half minute saving per probe, this adds up to 45 hours or six days work.

“By identifying features much faster, we could provide feedback to the client much sooner, in turn allowing us to carry out the required additional investigation of features to provide more detail. This meant foundations and any remedial works could be designed on a plot-by-plot basis, delivering significant time and cost savings to the client.”

Kelly says that client was very impressed with the speed and efficiency of the investigation, which has put Southern Testing in a good position for winning future work. “The approach is making us more competitive,” he confirms.